Stress quantification in a composite matrix via mechanophores

Abstract

Stress concentrations in polymer matrix composites occur due to non-uniform loadings which develop near the interface between the matrix and reinforcement in a stressed composite. Methods to better understand the evolution of this stress concentration are required for the development of advanced composites. Mechanophores, which are stress responsive molecules, can be embedded into the polymer matrix and used to quantify the local stresses in a loaded composite. In this work, single particle model composites were fabricated by combining functionalized glass particles embedded into a silicone/mechanophore matrix. Confocal microscopy was then used to measure the mechanophore activation in situ during mechanical loading. The fluorescence intensity was correlated to maximum principal stress values obtained from a finite element analysis (FEA) model of the system utilizing an Ogden hyperelastic model to represent the elastomer. By calibrating stress to fluorescence intensity spatially, quantitative stress measurements can be obtained directly from fluorescent images. To validate this technique, calibrated stress values for a two-particle composite system were compared to a FEA model and good agreement was found. Further experiments were performed on silicone matrix composites containing short cylindrical particles oriented with their major axis parallel or perpendicular to the stretching direction. To demonstrate the versatility of the single particle intensity/stress calibration approach, maximum principal stress values were mapped on the fluorescence images of the cylindrical experiments. This technique has potential to quantify stress concentrations quickly and accurately in new composite designs without the use of FEA models or differential image correlation.

Quantifying Localized Stresses in the Matrix of a Fiber‐Reinforced Composite via Mechanophores

“…To quantitatively assess the stresses within the system, we used a previously developed calibration approach. [ 40 ] This calibration method allows quantitative measurements of stresses developed in an MP‐containing sample assuming fixed imaging conditions and mechanophore content between samples. MP activation/stress calibration was achieved by determining a correlation between the experimental fluorescence micrographs at varying loadings and the calculated stress distributions predicted from the FEA model at corresponding strains.…”

“…Utilizing a previous calibration method, [21,40] which established a link between the intensity of experimental MP activation and the stress calculated by FEA of the similar material system, it was possible to measure stress based on MP activation. A comparison of the intensities of two systems -a single glass particle-reinforced composite and a single nylon fiber-reinforced composite-each with a matrix composed of 0.6 wt% SPN functionalized PDMS was carried out to employ this calibration.…”

“…The observed fluorescent intensity images were correlated with the maximum principal stresses from FEA. [ 40 ] The Ogden hyperelastic model was used to represent the polymer matrix in FEA. This calibration, correlating maximum principal stress ( σ m ) to intensity ( I ), enables direct quantification of the stresses in the system.…”

“…[43] In our previous studies, we demonstrated a systematic methodology to calibrate the stresses in polymer composites by correlating calculated stresses from FEA with experimentally observed MP fluorescent activation intensities of the same material system. [21,40] The calibration was accomplished by embedding stiff spherical silica particles into an SPN-functionalized silicone elastomeric matrix. When loaded, the rigid inclusion caused stress localizations in the matrix that exceeded the fracture strength of the matrix.…”

“…Our work has previously utilized MPs for stress sensing. [21,27,28,39,40] One major issue with the commercialization of MPs is quantifying MP activation and relating it to stress. In an effort to quantify the MP activation, Celestine et al carried out a fracture experiment on MP-functionalized rubber toughened PMMA.…”